This disclosure describes a unique internal combustion engine exhaust manifold and low cost manufacturing process for making it.
The manifold is lined with a ceramic pre-form which, on the inside where it is in contact with the exhaust gases, is coated with a thin coating of SiC which is known for its toughness and its high temperature resistance. On the inside (ie, in the middle or in the middle layer) of the pre-form there is an insulating medium. The entire pre-form is covered with steel or aluminum so from the outside it looks like a normal cast exhaust manifold.
The invented exhaust manifold has three distinct advantages:
1. Because of its low heat inertia it allows the exhaust gases to heat up the catalytic converter more rapidly. There are three reasons why it works in this fashion.
a. The silicon carbide shell is thin and lightweight and does not require as much energy to heat up as would a cast or fabricated steel manifold.
b. Unlike stainless steel or a high nickel alloy steel, silicon carbide (SiC) has excellent chemical and heat resistance.
c. A mineral insulation retains the heat in the inner liner of the manifold not allowing it to cool, particularly when the engine is heating up.
2. Because the manifold is so well insulated it is not necessary to have a protective heat shield on the outside of it. This saves the space and the expense of the heat shield while still protecting the temperature sensitive parts inside the engine compartment from the hot exhaust manifold.
3. Because the manifold contains layers of different types of material it does not transmit noise effectively. It does not resonate as much as thin walled fabricated steel manifolds do. On an automobile this reduces the need for auxiliary sound attenuation material such as the sound absorbing blanket on the cowl of the car.
Making an exhaust manifold with low thermal inertia has been a difficult and challenging endeavor. A former invention, U.S. Pat. No. 5,419,127 (the entire contents of which are incorporated herein by reference in their entirety as if fully rewritten hereat), assigned to Soundwich, Inc., describes a low inertia manifold using ceramic beads and a thin interior liner of high alloy metal, which would be in contact with the exhaust gases. This achieved the light-up goals, but even using high nickel alloys, like inconel, one could not obtain sufficient chemical and thermal resistance to provide sufficient durability. Because the ceramic beads provided so much insulation the inside skill became excessively hot causing the interior liner to decompose more rapidly.
The same is true with insulated fabricated stainless steel manifolds. Although they heat up quickly, faster than cast manifolds, they cannot be relied upon for durability.
If this were viewed as a purely metallurgical problem, it could probably be solved using expensive alloys; however, the cost bogey (in 1997 prices) for a fast light off exhaust manifold for a V-6 engine, is only $25-30 per manifold (two required for each engine). This would no where nearly cover the cost of a high alloy material selection.